linux-kernel - Re: console output duplicated when registering additional consoles

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Date:   Mon, 18 Nov 2019 13:38:04 -0800
From:   Jonathan Richardson <jonathan.richardson@...adcom.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>, gregkh@...uxfoundation.org,
        jslaby@...e.com, sergey.senozhatsky@...il.com,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        Scott Branden <scott.branden@...adcom.com>,
        Ray Jui <ray.jui@...adcom.com>,
        Srinath Mannam <srinath.mannam@...adcom.com>
Subject: Re: console output duplicated when registering additional consoles

On Thu, Nov 14, 2019 at 8:33 PM Sergey Senozhatsky
<sergey.senozhatsky.work@...il.com> wrote:
>
> Gosh, that part of printk is really complex.
>
> On (19/11/14 10:57), Petr Mladek wrote:
> > For a proper solution we would need to match boot and real
> > consoles that write messages into the physical device.
> > But I am afraid that there is no support for this.
>
> Wouldn't those have same tty driver?
>
> ---
>
>  kernel/printk/printk.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index f1b08015d3fa..a84cb20acf42 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2690,6 +2690,19 @@ static int __init keep_bootcon_setup(char *str)
>
>  early_param("keep_bootcon", keep_bootcon_setup);
>
> +static bool known_console_driver(struct console *newcon)
> +{
> +       struct console *con;
> +
> +       for_each_console(con) {
> +               if (!(con->flags & CON_ENABLED))
> +                       continue;
> +               if (con->device && con->device == newcon->device)
> +                       return true;
> +       }
> +       return false;
> +}
> +
>  /*
>   * The console driver calls this routine during kernel initialization
>   * to register the console printing procedure with printk() and to
> @@ -2828,6 +2841,9 @@ void register_console(struct console *newcon)
>         if (newcon->flags & CON_EXTENDED)
>                 nr_ext_console_drivers++;
>
> +       if (known_console_driver(newcon))
> +               newcon->flags &= ~CON_PRINTBUFFER;
> +
>         if (newcon->flags & CON_PRINTBUFFER) {
>                 /*
>                  * console_unlock(); will print out the buffered messages

Thanks. It also needs to be cleared when the second console driver is
registered (of the same type, boot or normal), not just when a normal
con replaces a bootconsole. A simple way of avoiding the problem I'm
seeing is to not even set the CON_PRINTBUFFER flag on my consoles. It
skips the replay and the output on all consoles looks fine. The flag
is only used by register_console(), although I don't think that is the
intended usage? There are no console drivers that do this.